Boat Builder Central - Builder Forums

TomTom time does seem to sneak up on us doesn't it. I'll wait to hear what the new prop does for you. The NRS is a pretty aggressive prop so I might have gone with a 14 pitch but lets see what happens with the 15. If you can keep track of rpm' s at full throttle and trolling speed and at cruise.

Happy Holidays!

Tom

Hey Tom - been having engine problems - finally found a corroded wire from a coil - been a nightmare. Don't want to hate on my Evinrude but it has been a mission to get it running again; one tiny electrical fault is so hard to locate.

Anyways I now have some numbers for my new prop from Ken at prop gods- I think you might be right that it was slightly too much. Can u let me know what u think of these numbers...

Top speeds... In kts

4900 - 27.2;
4950- 29.5
4900 - 28.8

4000- 24.4
2300 -7.9 and 6.2
2200 -7.2
2000 - 5.7
1000 - 4.6
1400 - 3.7

Hi Tom those numbers look low with the Fitch 90 and a 6000 rpm top end. I have been traveling yesterday and today so give me a day or two to recuperate and I will get back to you. Only question I have right now is what is the brand and size of your new prop, diameter and pitch.

It is an NRS4 in 13.5 x 15 pitch which is what Powertech suggested to Ken at Propgods; initially he wanted to go with the PTR4 in 13.5x14 but it was unavailable.

He says that the NRS4 is not available in a lesser pitch.

Rennet my criteria was to get 7.5 knots with the lowest rpm possible; but I am worried that this prop might damage or overstrain the engine?

He has suggested I try lifting the engine slightly - not sure it will make that much difference.


Your help well appreciated as always.

Let's start out. You are only getting 4900 rpm's, can you say engine damage. The Fitch is designed to run at 5500-6000 rpm's. The first thing we need to do is find a prop that will drop the pitch down to 12 or 13. I would like you to try the RED4 in a 13 pitch. This should take you up to the 5700 range, if it doesn't go to the 12 pitch. It may give you a little bit of bow lift, but nothing you can't control with your trim and tilt controls. The very first builder of the C19 finalized on a Stiletto, which this prop is similar to, and topped out at 36.5 kts with his 115 Yamaha. With the proper rpm's you should be in the 30.4-32.2 range and still able to troll at 7. The first builder trolled for marlin and sailfish and other offshore fish.

Hopes this helps. Again try for a 13" prop and try it 15 is way to much.

Tom

The RPMs are so much lower than maximum at the high end and so much higher at the low end, I suspect that it is not a propeller problem but probably something else like a clogged injector not giving enough fuel to one of the cylinders or another problem causing fuel starvation.

6-7 knots should come at about 1000 RPMS or so most likely.

I think you will frustrate yourself switching out props repeatedly.

Check cylinder compression first, then spark, and then fuel injection. Hopefully it is one of those three.

Browndog - thanks for your input. Lets say that I put the old aluminium prop back on and the rpm went up to say 5250 - which is about what it was before the change, would you still suspect the same engine faults or would that then rule that out as a possible problem?

Even with the aluminum prop you were well below maximum RPMs. My bigger concern is how high you have to rev the motor to go at 6-7 knots which is in the displacement speed range. Something is not right. It is running just well enough to mask something else. Since you don't have motor alarms going off I suspect it is something related to fuel. Has this motor sat along time without running much? That usually causes gunk to clog stuff up. It is pretty easy to check compression and spark to each cylinder. Start there and then look at the fuel injection or throttle would be my guess.

I had a similar problem on my flats skiff right after I bought it. Couldn't get the RPMs up and top speed was low. Tried switching propellers. Not much change. Sure enough bad gas and clogged injectors because the previous owner hadn't run the motor much over a long time.

I agree with Browndog.

I would also look at those things also before changing the prop. Hadn't about those affecting the motor. But with things going on like the different speeds at 2300 rpms I would take a look at the motor.